Electronic components such as semiconductor elements for optical communication are mounted on, for example, an electronic component housing unit. The electronic component housing unit is attached to and mounted on a mounting board. For example, a package described in Patent Literature 1 can be used as such an electronic component housing unit.
The package described in Patent Literature 1 includes a package board (substrate) on which a semiconductor integrated circuit chip (electronic component) is mounted, a via-hole (connection conductor) that extends from a top face to a bottom face of the package board, a signal line metal film (wiring conductor) that is disposed on the bottom face of the package board, and a ground metal film (ground conductor) that is disposed on the bottom face of the package board and that is located with the signal line metal film interposed therebetween. The package described in Patent Literature 1 can be fixed to a mounting board for use.
However, when the package described in Patent Literature 1 is mounted on the mounting board, it may be difficult to perform impedance matching. Specifically, a large difference in ambient dielectric constant is caused between a region of the wiring conductor located just below the substrate and a region of the wiring conductor drawn out from the side face of the substrate. The reason is as follows. In the region just below the substrate, the substrate is located above and the mounting board is located below. That is, dielectrics are present on both sides in the vertical direction when seen from the wiring board. On the contrary, in the region drawn out from the side face of the substrate, the mounting board is located below. That is, a dielectric is present on only one side in the vertical direction when seen from the wiring board. Accordingly, it is difficult to perform impedance matching between the regions.
Patent Literature 1: Japanese Unexamined Patent Publication JP-A 4-336702 (1992)